FIG. 1 (Prior Art) is a symbol of a thyristor 1. A conventional thyristor is a three terminal semiconductor device that has four layers of alternating N type material and P type material. FIG. 2 (Prior Art) shows an example in which the thyristor device 1 has an P-N-P-N structure. A cathode electrode 2 is coupled to the N type material on one side of the device and an anode electrode 3 is coupled to the P type material on the other side of the device. A gate electrode 4 is coupled to the P type layer closest to the cathode. The structure has three PN junctions, serially named J1, J2, and J3, from the anode electrode side of the device. Operation of the thyristor device is explained in terms of a pair of tightly coupled bipolar junction transistors, arranged in a self-latching way, as illustrated in FIG. 3 (Prior Art). One bipolar transistor 5 is an NPN transistor whose N type emitter is coupled to the cathode electrode. The other bipolar transistor 6 is a PNP transistor whose P type emitter is coupled to the anode electrode. The thyristor is considered to operate in one of three modes: 1) in a reverse blocking mode, 2) in a forward blocking mode, and 3) in a forward conducting mode.
If the cathode electrode has a positive voltage with respect to the anode electrode, then no current flows from cathode to anode because either the J3 junction and/or the J1 junction is reverse biased. These two PN junctions can be thought of as a series connected pair of diodes that are reverse biased. This is referred to as the reverse blocking mode. Supplying a triggering pulse into the gate has no effect.
If the anode has a positive voltage with respect to the cathode, but no voltage is applied to the gate electrode with respect to the cathode electrode, then the J1 and J3 junctions are forward biased, while the J2 junction is reverse biased. Due to the J2 junction being reverse biased, there is no conduction and the thyristor is off. This is referred to as the forward blocking mode. The anode-to-cathode voltage is applied in a direction that could cause conduction were there to be current flow across the J2 junction, but the thyristor has not been triggered. At some voltage, the electric field at the J2 junction grows so strong that the J2 junction starts to breakdown and a small amount of avalanche current begins to flow, but the amount of current is not adequate to turn on transistor 5.
If the anode-to-cathode voltage is increased beyond the forward breakdown voltage V(BO)F of the thyristor, then the magnitude of the avalanche current reaches a triggering current, which causes transistor 5 to turn on, and causes the thyristor to start conducting. This is referred to as the forward conduction mode.
If a positive voltage is applied to the gate electrode with respect to the cathode electrode, then the onset of this avalanche breakdown of the J2 junction occurs at a lower (but still positive) anode-to-cathode voltage. The anode-to-cathode voltage at which the thyristor turns on is therefore dependent upon the gate voltage (gate-to-cathode voltage). This positive gate voltage that causes the thyristor to turn on (due to avalanche breakdown of the J2 junction) can be caused by injecting a momentary current pulse 7 into the gate electrode 4. Whether the triggering on is said to occur due to the gate voltage pulse or due to the current of the pulse, the triggering effect of triggering the thyristor on is the same in that avalanche breakdown of the J2 junction is started.
Once avalanche breakdown of the J2 junction has occurred and the thyristor is on and conducting current from the anode to the cathode, the thyristor remains latched in this on state and the thyristor continues to conduct, irrespective of changes in the gate voltage, until either the anode is no longer forward biased with respect to the cathode, or until the current through the thyristor (anode to cathode) is less than a holding current IH. Once the thyristor has been triggered, removing the triggering current does not turn off the thyristor. If the anode is positively biased with respect to the cathode, then the thyristor cannot be turned off unless the anode current falls below the holding current IH. The thyristor can, however, be switched off if an external circuit momentarily causes the anode to be negatively biased with respect to the cathode.
Very large and expensive thyristors are used to switch high voltages in high power applications. Due to the limited maximum forward voltage drop (anode to cathode) that can be put across a thyristor before it conducts too much current and fails, many thyristors are typically assembled together in series in what is referred to as a stack. Each thyristor of the stack therefore only has to handle part of the overall high forward voltage drop across the stack. By controlling the gate voltages of the individual thyristors in the stack, the stack can be made to operate as a single high voltage and high power switch that either conducts from one end of the stack to the other, or that does not conduct. Thyristor stacks are, for example, used in megawatt scale AC-to-DC and DC-to-AC power conversion, such as for example where the high DC voltage is a voltage on a high voltage DC power transmission line. If the magnitude of the high voltage DC were to momentarily pulse high, such as due to the power line being struck by lightning, then an intolerably high transient voltage might be momentarily placed across the thyristor stack (the anode voltage is at too high a positive voltage with respect to the cathode), causing excessive current flow through the stack if the thyristors were on, and causing localized overheating and failure of the stack. To avoid this, and to protect the thyristors of the stack from such a transient overvoltage condition, an overvoltage protection device referred to as a BOD (Break Over Diode) device is used. BOD devices are coupled to the thyristors of the stack in such a way that the BOD devices turns on the thyristors before excessive over voltage across the thyristors can damage the thyristors. The BOD overvoltage protection device detects the high voltage condition and turns on, thereby causing a gate current to flow into the gate of each thyristor, and thereby turning on the thyristor.
A BOD diode is a thyristor whose gate electrode is not brought out of the device for external connection. The BOD diode has an anode electrode and a cathode electrode, but the gate electrode of the device is not brought out. The BOD diode is not triggered on by an externally applied pulse of gate current as described above in the case of a conventional thyristor, but rather the BOD diode is triggered on by the onset of avalanche current that is generated within the BOD device itself. Operation of the BOD diode is therefore explained using the terminology employed above in connection with the self-latching bipolar transistor structure of FIG. 3. In a forward blocking mode, as the anode-to-cathode voltage increases to the V(BO)F, the J1 and J3 junctions are forward biased but the J2 junction is reversed biased. A depletion region forms at this reverse biased J2 junction, and the resulting separation of charge at the junction gives rise to a localized electric field. The strength of the electric field grows to the point that covalent bonds of the material at the J2 junction are broken, and an avalanche current flows, with generated electrons being pulled to the relative positive potential of the anode, and with generated holes being pulled the opposite direction to the relative negative potential of the cathode. The hole flow through the P type layer between J2 and J3 results in a voltage drop across the material of the P type layer. If the avalanche current is of adequate magnitude, then the voltage drop exceeds 0.7 volts, resulting in the base-to-emitter voltage of the NPN transistor 5 (see FIG. 3) exceeding 0.7 volts. This causes the NPN transistor 5 to turn on. The NPN transistor 5 turning on pulls a base current out of the base of the PNP transistor 6. PNP transistor 6 turns on, and supplies current from its collector into the base of the NPN transistor 5. The two transistors are therefore latched on in a self-latching way, with each one supplying a base current to the other. Once the BOD diode has triggered itself on in this way, it will remain on unless either the anode is no longer forward biased with respect to the cathode, or until the current through the BOD diode falls below the holding current IH.
In an overvoltage protection application, the BOD diode is coupled in parallel with a power thyristor of a stack, such that if an excessive forward voltage develops across the power thyristor, then the BOD diode undergoes breakover and turns on, thereby conducting a current. This current is supplied as a triggering pulse to the thyristor, so that the thyristor is turned on. Accordingly, when the forward voltage across the thyristor reaches an adequately high voltage, the breakover diode supplies a triggering current to the thyristor and causes the thyristor to turn on. When the thyristor turns on, the forward voltage drop across the thyristor decreases. The thyristor is therefore protected from an over voltage condition.
FIG. 4 (Prior Art) is a cross-sectional diagram of a conventional bulk BOD diode 8. The PNPN thyristor structure is evident. Whereas the anode electrode 3 of the PNPN structure of FIG. 2 is illustrated on the top, the anode electrode 9 of the BOD diode of FIG. 4 is illustrated on the bottom. The P type substrate layer 10 is the first P type thyristor layer, the N− type substrate layer 11 is the second N type thyristor layer, the P type base region 12 is the third P type thyristor layer, and the N+ type region 13 is the fourth N type thyristor layer. Layers 10 and 11 are both of substrate silicon, so the BOD is referred to here as a “bulk” BOD. Reference numeral 14 identifies the cathode electrode. The BOD device has guard rings 15 and 16, and a peripheral shallow channel stopper 17.
FIG. 5 (Prior Art) is a diagram that shows the electric field along line A-A′ in FIG. 4 just before the onset of avalanche breakdown. The peak of the electric field is at the J2 PN junction. The J2 PN junction is between the N− type base layer 11 and the P type base region 12. When the electric field strength at this point is high enough, then the avalanche current begins to flow. If the magnitude of current flow from the junction laterally under the emitter region 13 across the resistance of the P type material of region 12 and to the cathode electrode 14 is adequately high, then the resulting voltage drop will reach 0.7 volts. This 0.7 volt drop amounts to a 0.7 base-to-emitter voltage on the NPN transistor, so the thyristor will be triggered on as described above.
FIG. 6 (Prior Art) is a diagram that shows the turning on of the BOD device 8 of FIG. 4. The BOD device has 8 has a reverse breakdown voltage of −200 volts to −300 volts. In the forward voltage condition, the solid line 18 in FIG. 6 represents current flow through the BOD device from anode to cathode. The dashed line 19 is the anode-to-cathode voltage across the BOD device. As can be seen, the BOD device begins to conduct appreciable current at a forward anode-to-cathode breakover voltage V(BO)F of about +450 volts. This is the time when the avalanche current in the BOD device has reached the triggering current. Thereafter the current increases and the voltage across the BOD device decreases. At time 0.6 microseconds the anode-to-cathode voltage across the BOD device has decreased to zero volts. The turn on time (TON) is the time from the time when the internal triggering current is reached at time 0.1 microseconds until the time when the anode-to-cathode voltage reaches zero volts. The turn on time (TON) is therefore 0.5 microseconds. Because a high transient overvoltage condition can cause a power thyristor of a stack to be destroyed in a short amount of time, a fast turn on BOD is desired for its thyristor-protection function.